An air conditioning system in which an indoor unit and an outdoor unit are connected to each other and exchange information therebetween in the serial communication style is described in JP-A-08-303842.
When communication is carried out between an indoor unit and an outdoor unit as in the case of the technique described in JP-A-08-303842, two communication styles have been hitherto adopted. According to one communication style, communication is carried out by using a dedicated communication line, and according to the other communication style, communication is carried out by using a power supply line as a communication line.
Accordingly, when existing facilities adopt the former style, only indoor units and outdoor units which are adaptable to the former style can be added. Likewise, when existing facilities adopt the latter style, only indoor units and outdoor units which are adaptable to the latter style can be added. Therefore, there is a problem that the existing facilities cannot be effectively used or an optional range for indoor units and outdoor units to be added is narrowed.